Jump to content

Epoxy creep


BillB

Recommended Posts

For the gunwales on my Moccasin boats, I cut pieces to make the 3/4 inch square strips then scarf the pieces together with un thickened epoxy to achieve the desired length. When new, the joint is a visible  line but over time the line becomes a line you can see and feel. The joint appears to be pulling apart lengthwise, could this be epoxy creep or am I doing something wrong?

 

This time I have pinned the join with a dowel through the scarf joint in an effort to prevent this creeping.

Anyone else experience this problem?

 

Bill

 

Update

 

see attached image

 

The dowel is not in previous gunwales but I am going to try it on this one.

 

post-387-0-19100700-1458779306_thumb.jpg

Link to comment
Share on other sites


The gunwales of my Lapwing are made of 3' pieces of teak scarfed and glued with epoxy, but using wood flour and some colloidal silica (B&B blend) as filler.  I clamped the pieces together with just finger clamps to avoid excess  squeeze out.   I can only conclude that your joints are starved of epoxy due to no thickening and maybe over-clamping?  I don't have any clever fix ideas.  Pictures might help.

Link to comment
Share on other sites

Epoxy doesn't really creep. There is some shrinkage, but the amount is so little it's hard to measure, compaired to many other adhesives. It's hard to tell what's going on with your rails, though I'd bet Dave may be on the right track. How much surface area is inside the joint, on the is 3/4" square stock, an 8:1 scarf? What were your thickening materials and was the end grain wetted out prior to the thickened goo application?

Link to comment
Share on other sites

I will defer to the above about the actual adhering, they are both undoubtedly more experienced than I.

If the seam is bugging you, though, I have a fix. You can use a thin bladed handsaw to cut through the joint, as if you were "kerfing" a joint together (which, you kinda are). Now, you make a tape dam, mix up a good, thinnish slurry, and force it into the kerf you just cut.

Easy, peasy, and good as new.

Link to comment
Share on other sites

If the seam is bugging you, though, I have a fix. You can use a thin bladed handsaw to cut through the joint, as if you were "kerfing" a joint together (which, you kinda are). Now, you make a tape dam, mix up a good, thinnish slurry, and force it into the kerf you just cut.

 

The gunwale is bent and under strain.  If you cut a kerf at the seam the gunwale will distort as the tension is released.

Link to comment
Share on other sites

I've done it on canoes. My wife and I tried for about ten years to wear out several canoes, and every river within one tank of gas...

I have repaired boats in situ many times.

The gunwale will indeed pull some kind of stunt, but I was assuming (uh-oh) the gunwale is firmly attached to the hull side and there is some other furniture to hold the boat shape.

If the gap opens too much, you can clamp a batten around the outside after re-smoodging the new little slice.

Maybe not super easy, but super easy for anyone who built their own boat. :)

Link to comment
Share on other sites

That's the way I do it, I use a sacrificial batten (1x2) for the initial part of the cut, then replace it with another, sliding the saw behind the new batten, to finish the cut. This batten will hold the now severed rail pieces, until you go her up. I usually cover the restraining batten with clear plastic packaging tape, so when it's goo time, I don't have to remove anything and the batten doesn't stick.

Link to comment
Share on other sites

I have experienced this with both thickened and un thickened epoxy. The gunwales are attached to the hull with thickened epoxy and show no sign of separation. The scarf joints are 8:1.  and are staggered on the hull. You may have hit upon the problem as starved, due to over clamping.

There is no crack at the join, it is as though the two pieces are pulling apart and the epoxy is stretching but not cracking. 

I refinished my boats every two years and can easily return the gunwales to their new condition. It is just the fact that it is happening that puzzles me.

Link to comment
Share on other sites

I agree, the epoxy doesn't creep the joined surfaces do, when under stress and over time. Epoxy we use on boats is stronger than the wood it is joining and I have never had an epoxy join fail, but have had the wood fail leaving wood still attached to the epoxy.

Link to comment
Share on other sites

  • 2 months later...

Is it possible the wood used wasn't as dry when you glued it up as it is now and the wood shrank over time as it dried out to equilibrium?

 

I've had stuff pull apart before when the epoxy wasn't fully cured before it was loaded (slow cure epoxy+cold temps+loaded joint=movement). It may look hard, and you can't dent it with a thumbnail, but it is not ready to handle a heavy load. The same thing could happen if your mix ratio was bad, as it may never cure right or be slow to do so, remain gummy for too long and allow movement of a heavily loaded joint.

Link to comment
Share on other sites

Heat is a byproduct of the chemical reaction and not all epoxies are thermolytic, at least compaired to the ones most use. Most of the marine epoxy we use are fine up to about 150 degrees, where clear and noticeable distortion can occur. It doesn't melt, but does lose about 40% of its physical attributes above these temperatures. In the real world, if you paint your deck or cabin top black and if you happen to live in Florida or Texas, you might see some ripples appear, maybe some seams or grain lines show up out of nowhere and bonded down and highly stressed pieces will distort under load. These piece will not just suddenly release, though when cool, loaded items may retain some of the distortion, particularly is cooled quickly (boarding wave for example).

 

This said, heat can be used to accelerate the chemical reaction. For every 20 degrees above the mid 70's, you'll half the cure times. I'm currently "cooking" a boat part in a homemade oven at 130 degrees. This slow cure epoxy usually takes a full 24 hours to cure (16 or more to sand), when in the mid 70's but I can start sanding after 6 hours, by post curing the goo. The benefit is I can use a slow hardener, so I have plenty of time to setup, apply and work the goo, but I can cook it off so I can continue working on it the same day.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

Supporting Members

Supporting Members can create Clubs, photo Galleries, don't see ads and make messing-about.com possible! Become a Supporting Member - only $12 for the next year. Pay by PayPal or credit card.




×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.